A diesel engine using a fuel injection valve of the two-stage valve opening type is used to suppress the instability of operation typically represented by the diesel knock caused during injection of fuel. A diesel engine of this nature is also useful for precluding the delay in firing timings and controlling the emission of nitrogen oxides.
A fuel injection valve of the two-stage valve opening type has two successive fuel discharge steps for each cycle of fuel injection. Fuel is injected into the associated one of the power cylinders of the engine in quantities respectively predetermined for the two fuel discharge steps. The quantities of the fuel to be discharged from the fuel injection valve assembly during the two fuel discharge steps are dictated by the amounts of movement of a needle valve element 42 incorporated in the fuel injection valve assembly. Specifically, the quantity of fuel to be discharged from the fuel injection valve assembly during the first or earlier fuel discharge step is determined by the preliminary valve lift PL of the fuel injection valve assembly, that is, the period of time for which the needle valve element 42 is moved from its initial axial position to a predetermined first critical axial position of the valve element. Likewise, the quantity of fuel to be discharged from the fuel injection valve assembly during the second or later fuel discharge step is determined by the full valve lift FL of the fuel injection valve assembly, that is, the period of time for which the needle valve element 42 is moved from the first critical axial position to a predetermined second critical axial position of the valve element. A fuel injection valves of this type is disclosed in, for example, Japanese Provisional Utility Model Publication (Kokai) No. 56-129568.
The two-stage opening fuel injection valve assembly disclosed in the Provisional Utility Model Publications No. 56-129568 uses two pressure springs which control the movements, respectively, of the needle valve element 42 for the two fuel discharge steps of each cycle of fuel injection. These two pressure springs are arranged in series and are seated on movable spring seat elements which are respectively associated with the pressure springs. One of the pressure springs contributes to the primary valve lift for the earlier fuel discharge step and the other to the main valve lift for the later fuel discharge step.
The prior-art fuel injection valve assembly has a drawback which results from the fact that the movable spring seat element contributing to the preliminary valve lift of the fuel injection valve assembly is intricate in shape. Extremely high techniques are thus required for controlling the dimensional accuracies of the spring seat element and the associated members and elements during machining, assembling and adjusting of these members and elements to provide a preliminary valve lift PL with a satisfactorily high degree of preciseness. It may be noted that generally more exacting control is required over the preliminary valve than over the main valve lift in a fuel injection valve assembly of the two-stage opening type.
Attempts have therefore been made to provide useful solutions to this problem of the prior-art fuel injection valve of the described construction. Typical of such attempts include those which have resulted in fuel injection valves disclosed in the Japanese Provisional Utility Model Publications (Kokai) No. 56-173757 and No. 61-184866. The prior-art fuel injection valve shown in each of these publications is characterized in that the pressure springs providing the preliminary and main valve lifts, respectively, are arranged in parallel. The parallel arrangement of the two pressure springs is useful for reducing the number of members and elements which affect the degree of accuracy of, particularly, the preliminary valve lift and alleviating the requirement for high techniques in machining, assembling and adjusting the component members and elements of the fuel injection valve assembly.
The improvement achieved by the prior-art two-stage opening fuel injection valve assembly taught in these publications are however not fully satisfactory.